Section 7963 Of Chapter 1. Tunnels And Mines From California Labor Code >> Division 5. >> Part 9. >> Chapter 1.

. All underground mines and tunnels with more than five men underground at one time shall have telephone or other communication systems to the surface in operation at any time there are persons underground. Such systems shall be installed in such a manner that destruction or removal of one phone or communication device does not make other phones or communication devices inoperative.